Aging is accompanied by increased prevalence of serum autoantibodies. One commonly detected autoantibody, IgM rheumatoid factor, is also found associated with rheumatoid arthritis and other autoimmune disorders. Much evidence indicates that this autoantibody plays a physiologic role in the immune response. The potential of human subjects to secrete this autoantibody and the age-related changes in its expression by human peripheral blood and bone marrow lymphocytes have been investigated. The size of this self-reactive B cell pool increases with advancing age. The lymphocytes expressing this potential are found predominantly in an early B cell subset in elderly individuals as compared to a more mature B cell subset in individuals with rheumatoid arthritis. Preliminary data show that IgM rheumatoid factors share idiotypes implying a common origin, possible from a single light chain gene or a closely related family of light chain genes.